For practicalization of nucleic acid therapy, an effective and safe nucleic acid delivery carrier is demanded. While virus vectors are nucleic acid delivery carriers with good expression efficiency, they have practical problems from the aspect of safety. Therefore, the development of non-viral nucleic acid delivery carriers that can be used more safely is ongoing. Among them, carriers using a cationic lipid are non-viral nucleic acid delivery carriers most generally used at present.
Cationic lipids are largely composed of an amine moiety and a lipid moiety, wherein the amine moiety showing cationicity and a polyanion nucleic acid electrostatically interact to form a positively-charged liposome or lipid membrane structure, which promotes uptake into cells and delivers the nucleic acid into cells.
As known cationic lipids generally and widely used, DOTAP and DODAP can be mentioned. These known cationic lipids form a positively-charged liposome or lipid membrane structure when combined with a phospholipid, which electrostatically interacts with a nucleic acid to be able to deliver the nucleic acid to the target cells (non-patent document 1).
Patent document 1 describes a cationic lipid containing a large amount of amino group so as to increase the amount of uptake into cells. The document argues that this cationic lipid shows high cellular uptake potency and acts on various cells having different cellular membrane compositions.
On the other hand, for a nucleic acid delivery carrier using a cationic lipid to exhibit a practical effect in vivo, the requirements of good in vivo kinetics, specifically high stability in blood, property to highly accumulate in the target such as tumor, and the like need to be fulfilled. Given the problem, Wheeler et al. showed that a lipid membrane structure containing a cationic lipid having pKa adjusted to around neutral shows a long lifetime in blood after intravenous injection, accumulates in tumor sites, and can mediate the expression of nucleic acid in the tumor sites (non-patent document 2).
While cationic lipids having improved in vivo kinetics have been developed as shown above, in view of the property of the nucleic acid delivery carriers that they generally introduce exogenous substances into cells, a large effect output from a small uptake amount is desired. That is, when a lipid membrane structure is used as a delivery carrier of an expression vector into cells, it is desired to increase the expression level per unit lipid membrane structure incorporated into the cells and enhance intracellular expression efficiency. To enhance the intracellular expression efficiency, it is necessary to also improve, besides in vivo kinetics, intracellular kinetics such as uptake process, escape from endosome and the like, nuclear membrane permeation and the like. Moreover, it is known that dissociation of nucleic acids from the carrier and enhancement of the bindability of transcription factors are necessary to facilitate intracellular transcription (non-patent document 3).
Examples of the facilitation of intracellular dissociation of nucleic acid from a lipid membrane structure include a compound wherein one amine moiety and two lipid moieties are bonded via disulfide (patent document 2) and a compound wherein one amine moiety and two lipid moieties are each bonded via a disulfide bond (patent document 3). It is described that these compounds have an effect to dissociate the nucleic acid interacting with the amine moiety from the lipid membrane structure by utilizing intracellular cleavage of the disulfide bond.
However, despite the progress in this field, the intracellular expression efficiency achieved by nucleic acid delivery carriers using conventional cationic lipid is not fully satisfactory.